This invention relates generally to an improvement in vapor compression cycle device performance, and more particularly, to a means and a method for the recovery of thermodynamic work of expansion in such a device.
In the thermodynamic cycle of a conventional vapor compression cycle device, working fluid is compressed, condensed, expanded and evaporated. Expansion of the fluid is typically accomplished through the irreversible process of flashing, which can be a source of cycle inefficiency in that available thermodynamic work of expansion (.rho.VdP) is not recovered. The present invention is intended to better recover this expansion work, thereby improving the cycle efficiency.
Most earlier attempts to recover this expansion work involved its conversion into mechanical work. These attempts may be typified by U.S. Pat. No. 4,170,116 (Williams) in which a flow of working fluid to be expanded is apparently intended to be employed in the rotation of a shaft.
In contrast, the present invention is intended to improve the thermodynamic efficiency of a device. Depending upon the application, this improved efficiency may be exhibited in two forms. In a first type of application in which there is a relatively constant operating temperature range as in a refrigerator, the practice of the present invention will result in a decrease in the compressor power required to provide a given amount of cooling. This is achieved by increasing the evaporator pressure, thereby also increasing the compressor suction pressure. In a second type of application in which there is a variable operating temperature range as in a heat pump, the practice of the present invention will enable the "pumping" of heat across a greater temperature range for a given amount of compressor work. Thus, in either application the resultant thermodynamic cycle efficiency is improved.
Certain refrigeration cycle designs currently improve their thermodynamic efficiency by recovering a portion of the available expansion work through a transfer of heat from expanding working fluid flowing in a capillary tube to vapor flowing in a compressor suction line resulting in a superheating of the vapor. However, only a relatively small portion of the available expansion work is recovered in such a device.
A device in which a portion of the available expansion work is apparently recovered by heat exchange with working fluid undergoing evaporation is described by W. F. Stoecker in "Improving the Energy Effectiveness of Domestic Refrigerators by the Application of Refrigerant Mixtures" U.S. Dept. of Energy Publ. ORNL/Sub-78/55463/1. The device described by Stoecker includes two discrete countercurrent heat exchangers disposed in a multicomponent working fluid flow path after a low temperature evaporator section and separated by a relatively high temperature evaporator section of the device. Working fluid is caused to flow from a device condenser through the two countercurrent heat exchangers prior to being flashed into at least a partial vapor phase conveyed to the lower temperature section of the evaporator. Thus, a portion of the available work of expansion is recovered through heat transfer in two separate heat exchangers operating across discrete elevated portions of the evaporator temperature range of a device.
As an example Stoecker describes a refrigerator application employing a 50% Freon-12/50% Freon-114 working fluid mixture in which the temperature range of the mixture while undergoing evaporation is approximately -20.degree. F. to -2.degree. F., with the external countercurrent heat exchangers operating approximately over the -12.degree. F. to -10.degree. F. and -5.degree. F. to -2.degree. F. portions of that range. The corresponding temperature drop for the working fluid liquid cooled in these two heat exchangers is approximately 90.degree. F. to 28.degree. F. and 28.degree. F. to 2.degree. F. respectively, with the fluid subsequently being flashed into at least a partial vapor phase at approximately -20.degree. F. prior to entering the low temperature evaporator section. Accordingly, the portion of available work of expansion from 2.degree. F. to -20.degree. F. is not recovered. In particular, in the example cited by Stoecker a beneficial extraction of approximately 20 BTU/lb. of heat is accomplished from the liquid between 90.degree. F. and 2.degree. F. However, in this example there remains available but unrecovered the additional enthalphy from 2.degree. F. to -22.degree. F. from Stoecker given parameters.
Additionally, the effective cooling potential of the evaporator defined as the cooling potential available for use in cooling a fluid of interest, such as air in a refrigerator unit, is lost in the device described by Stoecker over the temperature ranges of -12.degree. F. to -10.degree. F. and -5.degree. F. to -2.degree. F. Furthermore, the device described by Stoecker exhibits a thermodynamically inefficient transfer of heat as exemplified by the above-noted use of -5.degree. F. to -2.degree. F. cooling potential to cool working fluid from 90.degree. F. to 28.degree. F.
Accordingly, it is an object of the present invention to enable the lowering of the average temperature at which the evaporator duty of a device is available.
Another object of the present invention is to enable an increase in the evaporator pressure, and thus the compressor suction pressure of a vapor compression cycle device.
Another object of the present invention is to provide a more thermodynamically efficient closer matching of device cooling potential with the temperatures of fluids to be cooled.
Another object of the present invention is to improve the performance of a vapor compression cycle device by enabling the recovery of the maximum available thermodynamic work of expansion in such a device for a given set of design parameters.
Still another object of the present invention is to provide a simplified device in which heat associated with the work of expansion is absorbed by working fluid being evaporated without a loss of the effective cooling potential of an evaporator over a given temperature range.